Florida Sen. Marco Rubio spent about 15 minutes with conservative radio host Lars Larson on Thursday defending a sweeping immigration bill he and seven of his colleagues unveiled just hours before.

When Rubio was done, Larson's first three callers complained that the bill would reward people who broke the law to be here, and they didn't believe the border would be secured as the senators promise.

The interview illustrated that Rubio, a Republican and favorite of the Tea Party, will have difficulty convincing conservatives who have helped kill past attempts at immigration overhauls that the time is right to back his effort.

The bipartisan group of senators called the Gang of Eight unveiled a bill Thursday that would represent the biggest change to the nation's immigration laws in 27 years. It would give the nation's 11 million unauthorized immigrants a chance to become citizens, add $6.5 billion to secure the nation's border with Mexico and fundamentally alter the nation's legal immigration system.

Rubio has become the chief salesman of the bill, appearing on seven TV talks shows on Sunday. But it is his appeal to conservatives and Republicans that may make the difference between success and failure.

At each of his six interviews with conservative radio hosts Thursday, he was questioned intensely about worries of repeating the failures of the 1986 bill that gave amnesty to the nation's estimated 3 million unauthorized immigrants. Back then, the bill called for securing the border, but an estimated 11 million more people have since entered or remained in the country illegally.

"Obviously, there are people who have very strong feelings and quite frankly, will never be supportive of the reform we're working on, or any reform for that matter," Rubio said. "But ultimately, I think they deserve to hear what we're working on, why we're working on it and hopefully at least some of them will consider why we're doing this."

Despite the skepticism, there are signs Rubio will at least get a hearing. Larson said Rubio was the ideal choice to pitch the bill.

Both said Rubio would struggle to win over their listeners. "I don't think it's going to sell," Banks said.

Rubio also appeared on Rush Limbaugh's radio show. When he last appeared in January, Rubio assured Limbaugh that he would walk away from the deal if it didn't secure the borders before allowing unauthorized immigrants to become citizens. On Thursday, he said he had.

Limbaugh grilled Rubio on why he would be embracing a bill that "Democrats are salivating over" and could spell "suicide" for the Republican Party. But his listeners also heard high praise.

"I happen to like Sen. Rubio very much," Limbaugh said after Rubio finished. "He's a force of nature, he's a force of energy and he is a genuine conservative and full-throatedly and full-heartedly … believes in it. The bill itself — I'm never going to understand it."

The radio hosts were in Washington for an annual gathering. But even as Rubio was speaking, organizers from the Federation for American Immigration Reform had a post on their website asking people to sign a petition to tell Rubio they oppose his embrace of "amnesty."

While selling the bill to Americans will be important, Rubio and the Gang of Eight will also have to convince conservative members of Congress standing in the way.

Rep. Lamar Smith, R-Texas, the former chairman of the Judiciary Committee, wouldn't even discuss the bill's chances in the House because he doubted it could pass the Senate. He said many members will struggle with the idea of forgiving unauthorized immigrants and will need more security along the border.

"I can assure you that we will be focusing on real border security," Smith said.

But other Republicans said that Rubio is just what the party needs right now. Sen. John McCain, R-Ariz., has been candid about the need to gain ground with Hispanic voters after GOP presidential nominee Mitt Romney garnered only 27% of the Hispanic vote in the 2012 election.

Lionel Sosa, a San Antonio-based media consultant who advised the campaigns of the last three Republican presidents, said that with the rise of the Tea Party, it's become more difficult in recent years to walk the line between courting Hispanic voters while appeasing the "very, very conservative elements" of the party on immigration.

But unlike the GOP presidential candidates of 2012, Sosa said Rubio's background as the son of immigrants, and his wide appeal among conservatives voters, provides him the best chance to walk that line.

"Ronald Reagan gave amnesty to 3 million illegal immigrants. George H.W. Bush used to say, 'Family values don't stop at the Rio Grande.' George W. Bush had a very moderate stance on immigration all along," Sosa said. "And I think Rubio's exactly on the right track. He's conservative enough, and everybody knows that, and yet he's understanding enough that he knows it must be solved for immigrants."

Sen. Mike Lee, R-Utah, was initially approached to join the Gang of Eight by Sen. Charles Schumer, D-N.Y., shortly after the 2012 elections. But the Tea Party favorite declined to join after he realized they were trying to do everything at once — legalization for unauthorized immigrants included.

Lee said Thursday he would rather pass bills that secure the border and modernize the nation's legal immigration system, and only then figure out how to handle the nation's unauthorized immigrants — a common sentiment among conservatives.

"The bill in its current form is not something that I can imagine myself supporting," Lee said. "But because there are so many things in it moving in the direction of what we need, I'm looking through it with a fine-toothed comb to figure out whether it could either be repaired or amended or whether aspects of it can be spun off into different pieces of legislation that I could support."